1. Field of the Invention
The present invention relates to a method for selecting a ferrite bead for a filter.
2. Description of Related Art
Ferrite beads are widely used as attenuators or suppressors of unwanted signals in electrical circuits. A ferrite bead is inserted into a filter of a power supply circuit containing useful signals as well as electromagnetic interference (EMI). The desired signals will pass through the filter while the undesired EMI will be attenuated by the filter. This is accomplished through an impedance of the ferrite bead, which is frequency sensitive. Direct and low frequency currents see only the conductor, and are unimpeded. The high frequency energy couples with the ferrite bead and impedance is developed which has inductive and resistive components. When a line is passed through a ferrite core, low frequency energy is transmitted with little loss, whereas higher frequency energy encounters the inductive reactance caused by the real part of the complex permeability of the ferrite bead. The inductive reactance reduces the conducted EMI current and introduces an insertion loss.
However, the ferrite bead has a characteristic of resonance, which causes a peak value in a frequency response curve of the filter, which can lead to the filter oscillating, disabling the filter.
What is needed, therefore, is a method for selecting a ferrite bead for a filter, avoiding a peak value in a frequency response curve of the filter.